Fix SimpleSubscriberPlugin

[ivanpauno]

• Do not override the subscribeImpl method taking a rclcpp::SubscriptionOptions object.
  Plugins that were not overriding it before won't notice that they have to be update.

That was done in #186.
To be more clear, the issue is that external plugins are usually inheriting from SimpleSubscriberPlugin and not from SubscriberPlugin.
e.g. this method was not called anymore after that patch, resulting in the compressed image transport subscriber plugin not being correctly initialized.

[ivanpauno]

@jacobperron @audrow could you review?

[ivanpauno]

That one is fun, I cannot understand how this code was working joy.

Now that I see the diff, I deleted a line without realizing and then added it back 😂.
Updated OP as that was not an issue.

[ivanpauno]

Now that I see the diff, I deleted a line without realizing and then added it back joy.

Reverted the accidental changes in a3d3d31.

[ivanpauno]

I realized this is neither ideal unfortunately, see 8e4fc83.

We should either do this, or delete the subscribeImpl() method not taking a rclcpp::SubscriptionOptions argument and break API.
I don't see a better alternative ...

[jacobperron]

ugh, yeah this use of inheritance is not very maintainable. Maybe we go forward with this workaround for Galactic and then break the API for Rolling by removing the subscribeImpl without the SubscriptionOptions (or changing the design more dramatically).

[audrow]

Looks good to me.

[ivanpauno]

@mjcarroll could you review?

[chapulina]

This introduced a clang warning, fix in #196.